1. Field of the Invention
The present invention relates to analog-to-digital converters (ADCs), particularly ADCs for use in mobile wireless devices, such as wireless telephones.
2. Description of the Related Art
Analog-to-digital converters are used in a wide variety of applications. One such common application is in a radio receiver that is to convert analog signals to digital signals. Such digital receivers are common in the area of wireless communications.
In addition, when communicating with a wireless device there are sometimes interference tones from other sources. For example, when utilizing a wireless receiver in a code division multiple access (CDMA) system there are sometimes interference tones from nearby mobile transmitters that are operating in an Advanced Mobile Phone System (AMPS). Conventionally, these interference tones have been suppressed by using one or more analog filters before a low dynamic range ADC or by using a digital matching filter after a high dynamic range ADC. Unfortunately, both of these techniques have significant drawbacks.
As to the former technique, analog filters often can require a great deal of power and can be expensive to construct. In addition, there are often significant problems in mixing analog and digital circuitry. Thus, the latter technique has become the preferred approach. However, this technique requires that the ADC have a sufficient number of bits of resolution (or sufficient dynamic range given a fixed bit resolution requirement) to permit the digital matching filter to properly reject the interference tone regardless of the strength of the tone.
Although using a digital matching filter to reject interference tones has become the preferred technique, the present inventors have discovered that achieving the requisite dynamic range requires more power consumption than is ordinarily necessary (i.e., when only a low-level interference tone is present, which is usually the case). Thus, what is needed is an ADC that provides sufficient dynamic range when a high-level interference tone is present, but that reduces power consumption as compared to the conventional ADCs used for this purpose.
The present invention addresses this need by providing a two-stage analog-to-digital converter in which the second stage is enabled and disabled depending upon the level of the interference tone detected.
Thus, in one aspect the invention is directed to an analog-to-digital converter that includes a first analog-to-digital conversion (ADC) stage connected to input a first analog signal and a second ADC stage connected to input a second analog signal produced by the first ADC stage. A tone detector enables the second ADC stage from a disabled state when a first condition indicating the presence of a high-level interference tone is satisfied and disables the second ADC stage from an enabled state when a second condition indicating the absence of a high-level interference tone is satisfied.
In accordance with the present invention, high conversion dynamic range generally can be obtained, for example, when a high-level interference tone is detected, thereby better enabling subsequent rejection of the interference tone. At the same time, because the second stage can be disabled when such increased dynamic range is not required (e.g., when only a low-level interference tone is detected), an analog-to-digital converter according to the present invention often can minimize power consumption. Reduction of power consumption is particularly important in mobile devices (e.g., wireless telephones), which are often battery-powered.
In a more particularized aspect, the interference tone detector preferably utilizes a Fast Fourier Transform to detect the interference tone. Such an interference tone detector is believed to identify, more accurately than other technologies, whether or not a high-level interference tone in fact exists.
In a further particularized aspect, the tone detector uses a different condition to determine whether to enable the second stage than it uses to determine whether to disable the second stage. Such a technique can prevent the occurrence of a ping-pong situation, in which the ADC switches back and forth between states.
In a further particularized embodiment, the analog signal input by the second ADC stage corresponds to a measure of quantization error in the first ADC stage. In this embodiment, the output of the second ADC stage may be subtracted from the output of the first ADC stage, thereby canceling the quantization error from the first stage.